Hi guys,
I made a 27K poly tire and see that when I modify the subD (in mesh mode and not subD mode)
It scales in close to real time but takes 9 seconds to finish once I let go… But doing so on the extracted control mesh is instant.
Undoing the scaling on the SubD takes 17 seconds.
Again doing the same on the mesh is instant.
can you post the file for us to test here?
Of course!
It’s this tire (I just had to throw a rim into the file for you, found it free online)
This file takes a long time to open. I think it makes the huge mesh while it does so.
It also gobbles up 5 GB ram while doing so… and drops down to 2, goes up to 3 and goes down to 1.2 when opened.
The file was big (18MB) so I saved small and that chisled away a tiiiny bit, and now it is even slower to open. Test it out to see.
The rim is just a sub 100K mesh and is not a subD so it should not affect anything. But test it out and have fun! Hope you find something to fix 
SubD Tire.3dm (18.1 MB)
Hi Jorgen - I see this - My guess of the moment is that the preview and the extracted mesh are both just scaling a mesh and that it is adding the scaled SubD to the document that takes all the time. On the other hand, the Undo… I am not sure why that also takes time.
-Pascal
It appears that creating the display information for the tread SubD object is slow. (Undo creates a new display information.) I’ve created a bug and will investigate why this is slow.
https://mcneel.myjetbrains.com/youtrack/issue/RH-60542
Great to hear, good luck and sorry for having to make such a big subd for you, I just had too try… 
While I am at it.
Try twisting the subD and then extract control mesh and twist that and then convert back to subd. In my experience the latter is faster.
Any idea why?
The display speed bug is fixed in Rhino 7.1 which will be available Dec 10, 2020. More details can be found in RH-61377
